Microwave ionization of H atoms: breakdown of classical dynamics for high frequencies

We report the first measurements of microwave excitation and ionization of excited hydrogen atoms for scaled frequencies n/sub 0//sup 3/..omega.. up to 2.8. Classical 3d calculations which directly model this 36.021-GHz experiment agree quite well for n/sub 0//sup 3/..omega..2. This supports theoretical predictions that as n/sub 0//sup 3/..omega.. rises above one quantal ionization threshold fields rise above those for the onset of classical chaos; however, the data continue to reveal local stability near certain rational frequency ratios that recalls classical behavior.